Gryphaea arcuta

Gryphaea arcuata is an extinct species of foam oyster, a bivalve mollusc in the family Gryphaeidae from the Early Jurassic of Europe.

It is commonly referred to in English folklore as the 'devils toenail' due to its supposed resemblance to the devil's 'cloven hoof'. There used to be a common folk belief that carrying one of these fossils could prevent rheumatism. The name "devil's toenail" is also used for some fossil species of the genus Exogyra, which is in the same family (Gryphaeidae) as Gryphaea.

These oysters lived on the sea bed in shallow waters, possibly in large colonies. The complete fossils consist of two articulated valves: a larger gnarly-shaped shell (the "toenail") and a smaller, flattened shell, the "lid". The soft parts of the animal occupied the cavity between the two shells, just like modern oysters. The shells also feature prominent growth bands. The larger, curved shell sat within the mud on the sea floor.

Bivalvia, or bivalves, are a class of aquatic molluscs found in both marine and freshwater environments. These organisms have laterally compressed soft bodies enclosed by a calcified exoskeleton made up of two hinged shells, or valves. Unlike other molluscs, bivalves lack a head, radula, and odontophore. Instead, their gills have evolved into specialized organs known as ctenidia, which function in both feeding and respiration. Common examples of bivalves include clams, oysters, cockles, mussels, and scallops, many of which inhabit saltwater, while others thrive in freshwater.

The majority of bivalves are benthic filter feeders, typically burrowing into sediment to avoid predators. Some, like scallops and file shells, can swim, while others, like mussels, attach themselves to rocks or other hard surfaces. Shipworms, a unique type of bivalve, burrow into wood, clay, or stone, where they live and feed. The bivalve shell, composed of calcium carbonate, consists of two similar valves joined by a hinge and ligament. This structure allows the shell to open and close without the valves separating. The shells are typically bilaterally symmetrical, with the hinge line marking the dorsal (upper) side and the lower edge being the ventral (underside) region.

Adult bivalves vary greatly in size, from under a millimeter to over a meter, though most do not exceed 10 cm in length. Bivalves first appeared in the fossil record during the early Cambrian period, more than 500 million years ago. There are about 9,200 known living species classified into 1,260 genera and 106 families, with around 8,000 species found in marine and brackish water environments.

Bivalves have a unique body plan that includes a blade-shaped foot and a vestigial head, lacking a radula. The valves are composed of either calcite (as seen in oysters) or a combination of calcite and aragonite, with some species forming an inner nacreous layer. The outermost shell layer, called the periostracum, is a thin, easily worn layer secreted by the mantle, which covers the animal's body and plays a crucial role in secreting the shell and hinge mechanism. The shell grows as more material is secreted by the mantle, and its concentric rings are used to determine the age of the bivalve. In some cases, a cross-section of the shell can reveal more precise growth patterns.

Bivalves, like oysters and scallops, that lie on one valve have lost their anterior adductor muscle, relying on a single central muscle to open and close their shells. These muscles are composed of striated fibers for quick actions and smooth fibers for sustained pulls. Most bivalves are filter feeders, using their gills to capture food particles from the water. However, primitive bivalves, like protobranchs, feed by scraping detritus from the seabed using tentacles around their mouths.

Bivalves are often compared to brachiopods, another group of shelled marine organisms. Although they look similar, their evolutionary histories are different. In bivalves, the two valves are positioned on the left and right sides of the body, while in brachiopods, they are located dorsally and ventrally. Brachiopods have a lophophore, a specialized feeding organ, and while they were more abundant in ancient seas, bivalves now dominate. The success of bivalves is partly due to their ability to burrow and evade predators, and their varied adaptations have allowed them to thrive in different ecological niches. While brachiopods were once thought to have been outcompeted by bivalves, research now suggests that their decline was more likely due to extinction events rather than direct competition.

Bivalves played a key role in marine ecosystems for millions of years, surviving numerous mass extinctions and continuing to diversify through time. Today, they remain one of the most successful and widespread groups of molluscs.